What Actually Triggers A Period?

The first day of bleeding counts as day one of the menstrual cycle.

Which makes sense because it's what you can actually see so it's a natural inflection point.

But the events that caused the bleed started much earlier in the previous cycle, and a period is the result of those events.

The life of the corpus luteum

After ovulation, the follicle that released the egg becomes the corpus luteum. The corpus luteum produces some estrogen and a lot of progesterone, which changes the estrogen-built lining in the uterus into a lining that could support an early pregnancy.

If pregnancy happens, there will be hormonal signals that keep the corpus luteum going for longer than its usual 12 to 14 days.

If pregnancy doesn't happen, the corpus luteum reaches the end of its natural lifespan and is broken down. Progesterone and estrogen fall to their lowest levels of the cycle.

That fall is the main signal that starts a period after an ovulatory cycle.[1,2]

Progesterone withdrawal doesn't simply make the lining suddenly release.

It changes what the cells in the endometrium are doing. Inflammatory pathways switch on. Immune cells gather. Prostaglandins and other signalling molecules are released. Enzymes begin breaking down the tissue around the small blood vessels.[1–3]

How the blood flows through the lining changes too. These changes help start tissue breakdown and dissolution, while also helping trigger the repair process.[1,2]

This is why you can't fully understand a period by looking only at the day the bleeding began.

An early bleed can mean ovulation happened early. It could mean the luteal phase was shorter than usual. It could be bleeding that happened without ovulation. Or it may come from somewhere else, such as the cervix, a polyp, a fibroid or a pregnancy-related cause.

A late period is mostly associated with pregnancy, but if we put that possibility aside for a moment, a late period normally means ovulation was late. It's ovulation and the corpus luteum that set the timing for the period.

The cycle doesn't simply count out a fixed number of days and then release the lining.

In an ovulatory cycle, the timing of the period depends largely on when ovulation happened and how long the corpus luteum lasted, which is normally around 12 to 14 days.

The bleed is just the visible part. Although for many women it can also be the hardest part with a lot of pain and discomfort, let's not forget about that.

But most of the hormonal story happened before you saw it.

This is also why cycle tracking works by the way. Whichever day you ovulate is the day that sets the timing. Your period arrives about 14 days after ovulation, not 14 days into your calendar. What varies between women, and between cycles, isn't those 14 days. It's how long it takes to reach ovulation in the first place. A 24-day cycle and a 34-day cycle can both be normal. The difference sits before ovulation, not after it.[4]

At Moodmama we believe you should be tracking your cycle. It's one of the most useful things you can know about your own body. These days there are many fantastic cycle tracking apps making this process both easy and educational.

This article is the first of a 10 part series about the period as the first phase of a 7 phase cycle. Read the other articles below and follow us on social media for more cycle education.